1. Field of the Invention
The invention relates to an electric furnace and more particularly a direct-current arc furnace having an exchangeable bottom electrode.
2. Description of the Prior Art
An electric furnace of the above-noted type is known, for example, from Swiss Patent Specification No. 452,730.
Progress in the development of semi-conductor components in recent years has given an impetus to an increasing use of direct-current arc furnaces in the iron and steel industry for smelting, predominantly for smelting electric-furnace steel.
The construction and mode of action of direct-current arc furnaces are known, for example, from the journal "Stahl und Eisen", 103 (1983) No. 3, of Feb. 14, 1983, pages 133 to 137.
For optimizing the electrical or thermal conditions, it has proven to be advantageous in a direct-current arc furnace to form the arc between one or more electrode(s) located above the melting material and the melting material itself. As the return line of the direct current, at least one electrode in the bottom of the furnace and in contact with the melt, namely the bottom electrode, is provided.
The bottom electrode is exposed to continuous, very high thermal stresses, for which materials having a high softening and melting point, for example graphite, are suitable. However, when carbon electrodes are used, the melt is carburized on the one hand. This is undesirable, however, in particular in the production of low-carbon steels. On the other hand, the carbon electrode is consumed, whereby the furnace bottom is weakened and the electric power transfer can be adversely affected.
According to the solution proposed by the above-noted Swiss Patent Specification No. 452,730, bottom electrodes are used, but have a zone also having the same chemical contents as the melt itself in contact with the metal. In this case, cooling takes place in the end zone, facing away from the furnace vessel, of the bottom electrode by convection with air, this end zone consisting of a metal having good heat-conducting and current-conducting properties, for example of copper. This is a so-called two-component bottom electrode.
The high thermal loading of the bottom electrode and the associated wear phenomena make it necessary to replace the bottom electrode from time to time, that is to say to remove the consumed electrode from the furnace hearth and to replace it by a new one.
This bottom electrode is constructed to be conically tapered in the direction of the vessel interior. This constructional shape makes it possible to install and remove the bottom electrode from outside the furnace vessel bottom.
However, this type of bottom-electrode removal is very expensive, for the following reasons:
The solidified melt residues remaining in the furnace and the metallic components encrusted in the region of the refractory material of the furnace bottom which is close to the electrode together form a mushroom-shaped coating of metal and slag. Without prior removal of this mushroom-shaped coating, however, it is impossible to remove the bottom electrode. But the removal work is labor-intensive and can be carried out only in the cooled furnace vessel which, in turn, creates a time delay for putting the furnace into operation.
In addition, for the possible ejection of the bottom electrode from the furnace bottom an ejection bolt is required which is placed centrally onto the electrode face and the force of which acts axially. But this ejection operation presupposes that the face is constructed to be level and orthogonal with respect to the axis of the bottom electrode. Due to erosion effects, however, the face, turned towards the interior of the vessel, of the bottom electrode is more or less deformed and uneven, however, and the ejection process is possible only under difficult conditions and with the risk of damaging the refractory bottom. Measures must also be taken for anchoring the ejection device in the interior of the vessel.
In addition, as disclosed in to Swiss Pat. No. 452,730, the mechanical mountings including the electric power supply cables must be released and removed before every replacement of the bottom electrode.